ESO Education and Public Relations Dept.

Text with all links is available on the ESO Website

4 August 1998

Guiding the Giant: New ESO Survey Provides Targets for the VLT

Giant astronomical telescopes like the ESO Very Large Telescope (VLT) must be used efficiently. Observing time is expensive and there are long waiting lines of excellent research programmes. Thus the work at the telescope must be very well prepared and optimized as much as possible -- mistakes should be avoided and no time lost!

Astronomers working with the new 8-m class optical/infrared telescopes must base their observations on detailed lists of suitable target objects if they want to perform cutting-edge science. This is particularly true for research programmes that depend on observations of large samples of comparatively rare, distant objects. This type of work requires that extensive catalogues of such object must be prepared in advance.

One such major catalogue -- that will serve as a very useful basis for future VLT observations -- has just become available from the new ESO Imaging Survey (EIS).

The Need for Sky Surveys

Astronomers have since long recognized the need to carry out preparatory observations with other telescopes in order to "guide" large telescopes. To this end, surveys of smaller or larger parts of the sky have been performed by wide-field telescopes, paving the way for subsequent work at the limits of the largest available ground-based telescopes. For instance, a complete photographic survey of the sourthern sky (declination < -17.50) was carried out in the 1970's with the ESO 1-metre Schmidt Telescope in support of the work at the 3.6-m telescope at the ESO La Silla observatory.

However, while until recently most observational programmes could rely on samples of objects found on photographic plates, this is no longer possible. New image surveys must match the fainter limiting magnitudes reached by the new and larger telescopes.

Modern digital, multi-colour, deep imaging surveys have thus become an indispensable complement to the 8-m telescopes. The new generation of imaging surveys will, without doubt, be the backbone of future research and are likely to be as long-lived as their earlier counterparts, which have served the astronomical community so well over the past decades. The new surveys are now becoming possible, thanks to the new, extremely light-sensitive CCD-mosaics mounted on wide-field telescopes.

The ESO Imaging Survey (EIS)

A very successful, major step in this direction has recently been taken at ESO. It concerns an imaging survey with the 3.5-m New Technology Telescope (NTT) at La Silla, aimed at defining targets for the first year of operation of the VLT. In addition to serving the future observers, this survey is also public, i.e., the resulting data are made available to all interested parties.

The project is known as the ESO Imaging Survey (EIS). It is supervised by a Working Group with members from the European astronomical community ([1]) that has been responsible for defining the survey strategy and for monitoring the progress.

It has been a major challenge to carry out such a public survey in the very short time available. The work by the EIS Team has involved the survey observations at the NTT, development of a pipeline to process the raw data, advanced data reduction, identification of large samples of astronomically "interesting" targets and, not least, the distribution of images and other survey products before the start of operation of the VLT.

To cope with the ambitious one-year timetable, a novel type of collaboration between ESO and the astronomical communities in the ESO Member States was set up. It has allowed to combine efficiently the scientific and technical expertise of the community with ESO in-house know-how and infrastructure. This model has been very successful and may well set the example for future surveys.

Science Goals of EIS

EIS is in many aspects a novel approach for large-scale, ground-based optical observations, in support of large-telescope science. The speed with which raw EIS data have been converted to deliverable products is quite unprecedented, given the nature and scope of this project. This is a key ingredient for imaging surveys, the main goal of which is to provide target lists for 8-m class telescopes.

EIS consists of two parts: a wide-angle survey ("EIS-wide") and a deep, multi-colour survey in four optical and two infrared bands ("EIS-deep").

EIS-wide covers four pre-selected patches of sky (spanning the R.A. range from 22h to 9h). The main science goals of EIS-wide include the search for distant clusters of galaxies and quasars. In addition, there are important spin-offs in terms of bright and distant galaxies, as well as new information about galactic structure and stellar populations.

The observations were conducted in 10 runs in the period July 1997 - March 1998. A total of 36 nights were used for this part of the project. The images obtained cover a total area of 17 square degrees in the near-infrared I-band, reaching limiting magnitude of I ~ 23 and, furthermore, an area of 1.7 square degrees in the B- (blue), V- (green-yellow) and I-bands to a comparable depth.

Altogether, the EIS data set consists of about 6000 science and calibration frames, totaling 96 Gbytes of raw data and over 200 Gbytes of reduced images and derived products.

Some results from EIS

On these images, over one million galaxies were detected and about 250 distant clusters of galaxies were identified, with estimated redshifts in the range 0.2 < z < 1.3 [2]. This is by far the largest sample of distant clusters of galaxies currently available.

In addition, white dwarfs, very-low mass stars/brown dwarfs and high-redshift quasar candidates were identified in the field that lies in the direction of the South Galactic Pole.

All the calibrated images and derived catalogs are now publicly available. They can be examined and/or retrieved through an interface in the EIS release WWW-page built in collaboration with the ESO Science Archive, a prototype for future distribution of data to the ESO community.

A photo of a 25 arcmin wide field from EIS is available on the web as ESO PR Photo 18/98; the two versions may be accessed via ESO PR 07/98.

Future surveys at ESO

The EIS project has been conceived as a pilot project for more ambitious, future wide-field imaging surveys to be conducted by ESO. Together, they will provide the basic framework and infrastructure for the gradual development of the required capabilities for pipeline processing, archiving and data mining.

By January 1999, the ESO/MPIA 2.2-m telescope at La Silla will start regular observations with a wide-field camera capable of imaging in one shot an area of the sky that is larger than the full moon. This telescope will be fully dedicated to wide-field imaging and will be approximately 6 times more efficient than is the NTT for imaging surveys such as EIS.

An even more powerful survey telescope is now planned for the Paranal Observatory, next to the VLT. A Memorandum of Understanding has recently been signed by the Director General of ESO, Professor Riccardo Giaconni and the Director of the Capodimonte Observatory (Naples, Italy), Professor Massimo Capaccioli. According to this, the Capodimonte Observatory will deliver to ESO a wide-field 2.6-m telescope, referred to as the VLT Survey Telescope (VST).

The VST will be over 12 times more efficient than the 2.2-m telescope for survey work. When it goes into operation some years from now, ESO will consolidate its front-line position in wide-field imaging capabilities.

Another survey, the DEep Near Infrared Southern Sky Survey (DENIS), is now being carried out at La Silla. It is a joint European project that is conducted at the 1-m ESO telescope by a consortium of 20 astronomical institutes.

More information

Further information about EIS is available at From this site, it is possible to visit the EIS release page and to browse through pictures of the distant Universe and of individual objects, some of which will be observed with the VLT in the future.


[1] The home institutes of the astronomers involved in EIS include the European Southern Observatory, Osservatorio Astronomico di Trieste (Italy), Leiden Observatory (The Netherlands), Institut d'Astrophysique de Paris (France), Max-Planck Institut fur Astrophysik (Germany), Astronomisk Observatorium (Copenhagen, Denmark), Istituto di Radioastronomia del CNR (Bologna, Italy), Landensternwarte Heidelberg-Kvnigstuhl (Heidelberg, Germany), DAEC, Observatoire de Paris-Meudon (France), ESA/ESO Space Telescope-European Coordinating Facility (Garching, Germany), Osservatorio Astronomico di Pino Torinese, Torino (Italy) and Osservatorio Astronomico di Capodimonte (Napoli, Italy).

[2] In astronomy, the redshift (z) denotes the fraction by which the lines in the spectrum of an object are shifted towards longer wavelengths. The observed redshift of a distant galaxy or quasar gives a direct estimate of the universal expansion (i.e. the `recession velocity'). Since this expansion rate increases with the distance, the velocity (and thus the redshift) is itself a function (the Hubble relation) of the distance to the object. The indicated redshift interval (0.2 < z < 1.3) corresponds to a distance interval of approx. 3,000 to 7,000 million light-years.


ESO PR Photo 29/98

This photo shows three views of a small field in the so-called EIS Patch-B. They were obtained during this survey in different colours: B - blue; V - green-yellow; I - near-infrared. At the centre is located a (candidate) cluster of galaxies at very large distance. This conclusion is based upon the different appearance of this cluster in the three frames: it is not seen in B; it is hardly visible in V and it is most obvious in I. This indicates that the galaxies in the cluster have very red colours. The effect is most likely due to high redshift (and therefore large distance) that has shifted the bulk of their emission from the visual to the near-infrared region of the spectrum. The other objects in the field -- which are nearer -- can be seen in all three frames.

Copyright ESO Education & Public Relations Department
Karl-Schwarzschild-Strasse 2, D-85748 Garching, Germany

ESO Education and Public Relations Dept.

10 June 1998

Text with all links is available on the ESO Website

ESO Press Photo 15/98

The VLT Sharpens its View

Following the very successful First Light Event in late May, the first 8.2-m Unit Telescope (UT1) of the ESO Very Large Telescope (VLT) has continued to demonstrate its great potential. These developments are reported on a dedicated webpage with new VLT Information.

The VLT and Paranal -- a fine combination

The advanced, computerized system that provides total optical control of the telescope has been further optimized and is now operated in a fully active optics closed-loop mode. This implies that the telescope functions automatically (much like a giant autofocus camera). During recent test observations, it continuously corrects the shape of the primary mirror and the positioning of the secondary mirror. The latter, a light-weight Beryllium mirror, is now providing so-called field stabilization at a rate of 10 times per second (10 Hz). This innovative system decisively improves the achievable image quality.

The site of the VLT Observatory, Cerro Paranal, was chosen after more than 8 years of careful testing by ESO, between 1983 and 1991. The meteorological conditions at this isolated spot in the Chilean Atacama desert were found to be among the best on the Earth for astronomical observations. This has been amply confirmed, both in terms of low atmospheric turbulence and high visibility in the infrared spectral region, because of the very low water content in the air over this extremely dry site.

The sharpest images yet

The stable atmosphere above Paranal, in combination with optimized, total control of the VLT UT1 optics, has now allowed some of the sharpest astronomical images to be obtained. In fact, during a period of particularly good conditions, images were recently taken with a quality even better than the excellent ones available at the moment of "First Light".

The present photo is a reproduction of a digital image that was obtained in the early morning of June 6, 1998. It shows the central part of the southern globular cluster Messier 55 (or NGC 6809) in the constellation Sagittarius. The exposure lasted 30 seconds and was made with the VLT Test Camera through an optical filtre isolating mostly red light (570 - 700 nm). It is here shown exactly as it was obtained ("raw image"), without any image processing. The brightness level in the reproduction has been set so that the fainter stars are well visible. The field shown is 83 x 83 arcsec; each pixel measures 0.0455 arcsec.

Images of a large number of stars over the entire field have been measured and consistently show a stunning image quality. The full-width-at-half-maximum (FWHM) is only 0.27 arcsec! Moreover, the images are very nearly round -- the measured image elongation is negligible at the 5% level (0.015 arcsec).

This capability used on earth would allow us to clearly see the two headlights of a car at a distance of no less than 1200 kilometres (the distance from Paranal to Santiago de Chile) -- an astonishing performance of a ground-based telescope. It confirms that the VLT will be able to take full advantage of moments of particularly good observing conditions at Paranal and may soon begin to look beyond current horizons.

Present work

The UT1 has now entered the "Commissioning Phase" during which all systems are thoroughly tested and further tuned, leading up to "Science Verification" observations in the second half of August 1998. Thereafter, the two main astronomical instruments, FORS and ISAAC, will be mounted at the telescope and thoroughly tested. The UT1 will receive the first "visiting astronomers" for regular observing runs from April 1, 1999.

Further images of astronomical objects from the VLT UT1 will be published at irregular intervals.

This is the caption to ESO PR Photo 15/98 [JPG, 304k]. It is also available in a high-resolution version [JPEG, 1.2Mb]. It may be reproduced, if credit is given to the European Southern Observatory.

            Copyright ESO Education & Public Relations Department
           Karl-Schwarzschild-Strasse 2, D-85748 Garching, Germany

First Astronomical Photos from the VLT UT1

First Light homepage


27 May 1998

Text with all links is available on the ESO web site

A Great Moment for Astronomy

VLT First Light Successfully Achieved

The European Southern Observatory announces that First Light has been achieved with the first VLT 8.2-m Unit Telescope at the Paranal Observatory. Scientifically useful images have been obtained as scheduled, on May 25 - 26, 1998.

A first analysis of these images convincingly demonstrates the exceptional potential of the ESO Very Large Telescope. Just one month after the installation and provisional adjustment of the optics, the performance of this giant telescope meets or surpasses the design goals, in particular as concerns the achievable image quality. Exposures lasting up to 10 minutes confirm that the tracking, crucial for following the diurnal rotation of the sky, is very accurate and stable.

It appears that the concept developed by ESO for the construction of the VLT, namely an actively controlled, single thin mirror, yields a very superior performance.

In fact, the angular resolution achieved even at this early stage is unequalled by any large ground-based telescope.

The combination of large area and fine angular resolution will ultimately result in a sensitivity for point sources (e.g. stars), which is superior to any yet achieved by existing telescopes on Earth.

The present series of images demonstrate these qualities and include some impressive first views with Europe's new giant telescope. After further optimization of the optical, mechanical and electronic systems, and with increasing operational streamlining, this telescope will be able to deliver unique astronomical data of the highest quality. The commissioning and science verification phases of the complex facility including instruments will last until April 1, 1999, at which time the first visiting astronomers will be received.

The full significance of this achievement for astronomy will take time to assess.

For Europe, this is a triumph of the collaboration between nations, institutions and industries. For the first time in almost a century, European astronomers will have at their disposal the best optical/infrared telescope in the world.

We can now look forward with great expectations to the realization of many exciting research projects.

The First Light Images

Images of various celestial objects were obtained with the VLT CCD Test Camera, some of which are included in a new series, First Astronomical Images from the VLT UT1.

None have been subjected to image processing beyond flat-fielding (to remove variations of the digital detector sensitivity over the field) and cosmetic cleaning. They all display the recorded image structure, pixel by pixel. A detailed evaluation with accompanying explanations is presented in the figure captions.

1. Omega Centauri Tracking Tests

This 10-minute image demonstrates that the telescope is able to track continuously with a very high precision and thus is able to take full advantage of the frequent, very good atmospheric conditions at Paranal. The images of the stars in this southern globular cluster are very sharp (0.43 arcsec) and are perfectly round, everywhere in the field.

2. The Quadruple Clover Leaf Quasar

This 2-minute exposure of the well-known Clover Leaf quasar, a quadruple gravitational lens in which the largest distance between two components is only 1.3 arcsec, was obtained during a period of excellent seeing (0.32 arcsec) measured with a seeing monitor at the top of Paranal. The recorded angular resolution of just 0.38 arcsec demonstrates near-perfect optical quality of the telescope.

3. The Central Area of Globular Cluster M4

This is a colour composite of a field near the centre of the nearest globular cluster. At a seeing of 0.53 arcsec, the blue exposure reaches magnitude B = 24 in only 2 minutes (at signal-to-noise ratio = 5) in a bright sky. A simple extrapolation shows that B ~ 28 would be reached in a 1-hour exposure in a dark sky. The large mirror surface of the VLT UT1 and its ability to produce very sharp images, ensures that faint objects may be observed extremely efficiently.

4. Fine Structure of the Butterfly Nebula

This beautiful colour picture is a composite of three exposures through broad-band blue, green and red filters, lasting a total of 25 minutes. It shows the great complexity of this planetary nebula. It also demonstrates the exceptional efficiency with which features of faint surface brightness can be recorded with the VLT. Strong radiation from a dying star in a binary system at the centre impacts on the surrounding material that has been thrown out earlier from the system.

5) High-velocity Ejecta in Eta Carinae

This fine picture was obtained during an exposure lasting only 10 seconds. It shows fine structures around this very active object in a detail never before achieved with any ground-based telescope. In the lower insert, a short exposure of the central Homunculus Nebula (seeing 0.38 arcsec) provides a clear view of the three-dimensional structure of this bipolar object.

6. The Dust Band in Centaurus A

An amazing amount of faint details is shown in this high-resolution exposure (0.49 arcsec) of the central dust band in the nearby, southern galaxy Centaurus A, obtained through a broad-band red filter and lasting only 10 seconds. The VLT Unit Telescopes will be able to image many other galaxies in similar detail.

7. The Energetic Jet in Messier 87

The First Light took place during the night of May 25-26, 1998. Following a short interval of reasonable observing conditions, less optimal atmospheric conditions were encountered. The present photo, a three-colour composite (ultraviolet, blue, green) of the central region of the giant elliptical galaxy Messier 87 in the Virgo Cluster, was obtained during this night.

8. Total Optical Control

The 8.2-m main and the 1.1-m secondary mirrors of the VLT Unit Telescopes are completely computer-controlled by means of an Active Optics system. In this way, the shape of the mirror can be optimized very quickly for a given observational purpose. This sequence of 9 images illustrates how the appearance of a stellar image at the focal plane is fully controllable. Fast and thorough optical adjustment ensures the best possible optical quality at all times.

9. Image Quality of the VLT

This diagram demonstrates that First Light specifications have been fully met and, more impressively, that the actual VLT performance is sometimes already within the more stringent specifications that were expected to be fulfilled only three years from now.

The final steps before "First Light"

The final, critical testing phase commenced with the installation of the 8.2-m primary (at that time still uncoated) Zerodur mirror and 1.1-m secondary Beryllium mirror during the second half of April. The optics were then gradually brought into position during carefully planned, successive adjustments.

Due to the full integration of an advanced, active control system into the VLT concept, this delicate process went amazingly fast, especially when compared to other ground-based telescopes. It included a number of short test exposures in early May, first with the Guide Camera that is used to steer the telescope. Later, some exposures were made with the Test Camera mounted just below the main mirror at the Cassegrain Focus, in a central space inside the mirror cell. It will continue to be used during the upcoming Commissioning Phase, until the first major instruments (FORS and ISAAC) are attached to the UT1, later in 1998.

The 8.2-m mirror was successfully aluminized at the Paranal Mirror Coating facility on May 20 and was reattached to the telescope tube the day thereafter, cf. ESO PR Photos 13a-e/98 and ESO PR Photos 14a-i/98. Further test exposures were then made to check the proper functioning of the telescope mechanics, optics and electronics.

This has lead up to the moment of First Light, i.e. the time when the telescope is considered able to produce the first, astronomically useful images. Despite an intervening spell of bad atmospheric conditions, this important event took place during the night of May 25-26, 1998, right on the established schedule.

ESO Education and Public Relations Dept.

13 May 1998

Text with all links is available on the ESO Website

Expectations Increase as VLT First Light Approaches

Two weeks before the moment of "First Light" of Unit Telescope no. 1 of the Very Large Telescope (VLT), the ESO Team at the Paranal Observatory reports good progress of the preparatory work.

The crucial optimization of the world's first, thin 8.2-metre mirror proceeds according to the established plan. It is thus expected that this important event will take place as foreseen, i.e. during the night of May 25-26, 1998.

If no unforeseen obstacles are encountered, the first scientific images will then be presented during a series of near-simultaneous Press Conferences in the ESO member countries on May 27. The photos will be published on the WWW the same day, together with explanatory texts.

In preliminary optical tests at the first VLT Unit Telescope (UT1), the initial adjustment of the active optics system that controls the telescope optics has demonstrated excellent results. In particular, the first tests have verified the fine optical performance of the 8.2-m primary mirror and of the complex control system that maintains the shape of this thin and flexible Zerodur mirror.

In short test exposures with the guide probe (the technical device that is used to steer the telescope) - i.e., not yet with the scientific CCD-camera that will be used for the First Light images - the telescope has been following the external seeing provided by the Paranal site. Image quality of better than 0.5 arcsec has been achieved routinely.

"We are pleased with the progress and confident that the telescope will live up to the expectations", says Riccardo Giacconi, Director General of ESO. "The team at Paranal is doing a great job."

For more details about the various media activities surrounding the VLT First Light event, please consult the First Light homepage.

A list of locations, times and contact addresses for the Press Conferences is available on the web.

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